There is no way to change the default, but it’s rare that you’d need to.

By default, both the input and output channel formats of a newly created audio track are set to ‘automatic.’ The automatic channel format adopts the format of the content routed into it, meaning that if the sound modules on a track output stereo content, the track will automatically assume the stereo format for both its input and its output.

In the event that a bus or track set to ‘automatic’ receives content in more than one channel format, it adopts the channel format that features the largest number of channels, or the project surround format for the currently selected platform. Content is automatically upmixed (or downmixed) as necessary when the channel format changes.

The net effect of the above is that by default, buses and tracks automatically adopt the minimum channel format required for their content. Some specific projects may benefit from overriding this behavior, but such projects are rare.